1. Field of the Invention
The invention relates to a brake control system for automotive vehicles of the type in which braking force is applied commonly to the right and left driving wheels of an automotive vehicle.
2. Brief Description of Related Art
Recent automotive vehicles have been equipped with anti-lock brake systems (ABS) which estimate a tendency of wheel lock during braking based on both vehicle speed and wheel speeds and control braking force to the wheels in order to provide more stable braking performance by eliminating possible skid.
Such an anti-lock brake system is structured in various types in regard to the number of wheel speed sensors and control channels, for example, a 4-sensor, 4-channel system type, a 4-sensor, 3-channel type and others as well. In a 4-sensor, 3-channel type of anti-lock brake system, employed in rear drive vehicles for example, front driven wheels are controlled through independent hydraulic control circuits based on wheel speeds detected by respective front wheel speed sensors, and rear driving wheels are, however, controlled through a common hydraulic control circuit based on consolidated data of wheel speeds from the rear wheel speed sensors. Such an anti-lock brake system is known in, for example, Japanese Unexamined Patent Publication No. 4-197864.
Further, anti-lock brake systems of a 3-sensor, 3-channel type or of a 2-channel type employing a cross-arrangement of pressure lines provide braking control commonly to the left and right driving wheels.
Because all of these anti-lock brake systems monitor the speed of each driving wheel, they permit more reliably what is called select-low braking control which refers to the fact that braking force is controlled to an appropriate level for either one of the driving wheels which tends to lock more easily than the other driving wheel or has tire torque smaller than the other driving wheel. The select-low braking control provides an reduction in yaw moment of the vehicle and, consequently, permits the driver to more easily maintain steering control for the stability of vehicle driving.
In cases where front-engine rear-drive vehicles are equipped with the automatic brake system of the type in which braking force is controlled commonly to right and left driving wheels, if the select-low braking control is executed when the vehicle is traversing a split surface road which imposes different road surface frictional coefficient on the left and right driving wheels (rear wheels), either one of the driving wheels suffering a road surface frictional coefficient smaller than the other will tend to be brought into a locked condition and the braking force to the wheels will be reduced consequently. As a result, because the remaining wheels are judged to have a tendency to be brought into a locked condition due to an increase in speed of the driving wheel suffering a higher road surface frictional coefficient, and hence an increase in vehicle speed estimated based on that wheel speed, braking force to each wheel is possibly decreased below a necessary level.